I have a Dillon RL 550B, and I have a couple of sets of Lee dies that I use with it - leftovers from my days with a Lee Pro1000. The Lee dies work great, however - I find that they are a little short for the Dillon toolhead. I think the threaded area on the Lee dies are not as long as other die manufacturers. They work, but there's not much thread left for the locknut to grip.
Your kits seem like they include everything. If you have multiple calibers, you might want to consider the Dillon toolhead stand. If you're a handy woodworker, you can make one.
You are right about the Lee powder die. You'll need the Dillon die if you're going to use the Dillon powder measure. Each caliber conversion kit comes with a cartridge-appropriate powder funnel that fits into the powder die.
Not sure about the sizing issue with the Dillon dies you mentioned. I have several sets of Dillon dies, and they all produce ammunition that runs flawlessly in my guns. They are a bit pricey, though. Redding dies are good too. Very high quality, and their regular die sets don't seem too pricey.
